This thesis shows that the physics of cold self-gravitation fluids such as dark molecular clouds is much richer than usually assumed. The segregation in a gravitational field of small grains towards larger bodies such as comets and planetoids cannot be simulated with traditional hydro- dynamical codes, but is possible with a super-molecular approach. Observations, linear and virial analysis as well as computer simulations suggest the possibility of the formation of substellar H2 bodies due to the combination of phase transition and gravity in cold regions, as fluids in a phase transition are gravitationally unstable, independent of the strength of the gravitational potential. H2 phase transition is reached easily during plane-parallel collapses if the initial temperature is ≤ 15 K.